Decal projection on complex shapes
Get an angle of your object &quot;front on&quot; by using one of the orthographoc angles (Numpad 1 for front, Numpad 3 for side, Numpad 7 for top, or Numpad 9 to flip any angle by 180deg), and then press U (unwrap), and select &quot;Project from view&quot; This will make your UV map resemble a &quot;snapshot&quot; from that angle.

After that, connect your image texture as normal. It comes out looking like this:

If you want to stop the image from appearing on the top and sides of the stairs. you can use the respective axes from the Normal Vector as a mix factor between your image and whatever other material (I used plain white for the example). The Normal vector doesn't like being recombined without a third axes, so you have to do it in 2 stages - one for each respective axis that you need &quot;erased&quot;: